The effect of the blowing engine depends, in a considerable degree, on the density of the air, and the velocity with which it enters the furnace. Mr Mushet is of opinion, that the area of the discharging pipe, and the compression of the blast, depend on the qualities of the coals employed. " A soft or mixed qualify of coal," he observes, " is more susceptible of combustion than either the splint or clod coal ; the consequences of which is, that unless the necessary compression of air is used, decomposition is too early accomplished, and the cokes become oxygenated by combustion in a greater ratio than for the carbonation of the metal. To avoid this, the column of air ought to be discharged, in the case of soft coals, under such a degree of compression as to resist entire decomposition in the ignited passage." From this cause, the density of the blast should vary from 2 to 34- pounds in the square inch. Mr Roebdck supposes, that, " with the given power, it is rather by a great quantity of air thrown into the furnace with a moderate velocity, than by a less quantity thrown in with a greater velocity, that the greatest benefit is derived in the smelting of iron stones in order to produce pig iron." " Chemically speaking,'i
says Dr Robison, " it seems to be the quantity, not the density of the air, which renders it effective in the iron furnace. Yet I can conceive a great quan tity of air sent through a furnace without effect, be cause, being spread through much of the materials, it consumes the fuel too slowly in the different parts of the furnace for raising the smelting heat, whereas the same quantity made more dense, by crowding it into one part of the fuel, will rapidly consume it, and give out all its heat in a very narrow space, and thus produces effects which cannot be produced in any other way." For some of the preceding observations, we have been indebted to a manuscript paper of the late Professor John Robison, which, we believe, was the last production of that distinguished philosopher. It is an answer to several queries of Mr Grieve re specting blowing engines ; and we hope to have it in our power to present it to our readers in a future part of our work. For farther information on the subject of this article, see Smeaton's Reports; Roe buck, Phil. Trans. Edin. vol. v. p. Si.; and Mushet, Phil Ming. vol. vi. p. 60, 113, 362. ('r)